aNational Center of Technology Innovation for Digital Construction, Huazhong University of Science and Technology, Wuhan 430074, China
bSchool of Civil and Hydraulic Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
cSchool of Materials Science and Engineering, Southeast University, Nanjing 214135, China
dState Key Laboratory of Materials Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
The construction of extraterrestrial bases has become a new goal in the active exploration of deep space. Among the construction techniques, in situ resource-based construction is one of the most promising because of its good sustainability and acceptable economic cost, triggering the development of various types of extraterrestrial construction materials. A comprehensive survey and comparison of materials from the perspective of performance was conducted to provide suggestions for material selection and optimization. Thirteen types of typical construction materials are discussed in terms of their reliability and applicability in extreme extraterrestrial environment. Mechanical, thermal and optical, and radiation-shielding properties are considered. The influencing factors and optimization methods for these properties are analyzed. From the perspective of material properties, the existing challenges lie in the comprehensive, long-term, and real characterization of regolith-based construction materials. Correspondingly, the suggested future directions include the application of high-throughput characterization methods, accelerated durability tests, and conducting extraterrestrial experiments.
Currently, the exploration of extraterrestrial bodies has become prevalent among major national space agencies and commercial aerospace companies. Over the past hundred years, human exploration has reached the Moon and Mars. Since the success of lunar orbiting, remote sensing data have been analyzed to infer information such as surface temperature [1], [2] and regolith properties [3]. With the development of aerospace technology, extraterrestrial landings and in situ detection have become possible. A more concrete understanding of extraterrestrial objects, particularly the performance of extraterrestrial regoliths, can be obtained. Previous visits to the lunar surface include the Apollo, Luna, Surveyor, Chang’E, and Chandrayaan missions. The Apollo program achieved manned lunar landing flights and onsite human inspections. Nearly 382 kg of lunar regolith samples, including basalt, breccia, plutonite, soil, core, and others, were brought to Earth for further scientific research [4]. Similarly, Chang’E-5 brought 1731 g of regolith, including shoveled and drilled samples. For Mars, Viking 1 was the first to land on “the Red Planet,” followed by a series of Mars rovers, such as Sojourner, Opportunity, Curiosity, Perseverance, and Zhurong [5]. In addition, exploration of exoplanets is planned for the future. Landing is not the end of exploration, and the establishment of infrastructure on an extraterrestrial body is a future goal. Future exploration plans, including construction missions, are emerging. Artemis plan [6] announced by the National Aeronautics and Space Administration (NASA) proposes the construction of a lunar base camp. Similarly, the construction of International Lunar Research Station is planned by the China National Space Administration (CNSA) [7] and Russia’s Roscosmos. Related techniques have been widely studied to satisfy the requirements of extraterrestrial construction missions.
In terms of extraterrestrial construction, in situ construction methods have become the most promising because of their sustainability, considering the huge economic cost of extraterrestrial transportation. Accordingly, the extraterrestrial regolith is the main raw material available. The preciousness of the lunar regolith samples hinders the use of destructive experiments for construction research. Various regolith simulants with different simulation goals have been prepared by different research institutions and countries based on the cognition of actual extraterrestrial regolith. Some have been developed for the simulation of specific samples, such as Olivine-Bytownite-1 (OB-1) for the Apollo 16 anorthositic regolith sample 64500 [8]. Others have been prepared for a certain type regolith, such as Tongji-1 (TJ-1) for lunar low-Ti mare regolith [9]. In addition, regolith simulations may be performed to simulate the engineering properties of the actual regolith, such as the low water absorption of Mojave Mars Simulant (MMS) [10]. Using the material basis provided by extraterrestrial regolith simulants, dozens of preparation methods for regolith-based construction materials have been proposed by different groups. According to the principle of formation, construction materials can be fabricated in two main ways: concrete curing and regolith sintering/melting. Concrete materials for extraterrestrial construction mainly include sulfur [11], biopolymer [12], [13], [14], geopolymer [15], [16], polymer [17], [18], silicate [19], [20], and magnesium [21], [22]. For sintering/melting materials, processes such as hot pressed sintering [23], microwave sintering [24], [25], [26], [27], and spark plasma sintering (SPS) [28] can be conducted inside the molds. They can also be produced by additive manufacturing processes, such as laser sintering/melting [29], [30], [31], [32], [33] and digital light processing (DLP) [34]. In addition to existing methods, various new forming processes have been proposed. The preparation of these materials for extraterrestrial construction offers a significant foundation for future research.
When it comes to the practical utilization in extraterrestrial scenarios, the properties of different construction materials should be evaluated and appropriate materials should be chosen in accordance with actual construction requirements. These requirements mainly arise from the extreme extraterrestrial environment, which may cause challenges for both the preparation and application processes. Specifically, extreme extraterrestrial environments include space debris, hard vacuum, low gravity, temperature fluctuation, a weak atmosphere, and extreme radiation. Extraterrestrial construction materials should provide safe and stable environment for astronauts when used to build extraterrestrial structures. Correspondingly, the requirements for the properties of construction materials can be divided into three categories: mechanical, thermal and optical, and radiation-shielding. The mechanical properties include strength, impact resistance, hardness, and fatigue behavior. Knowledge of the mechanical properties of materials can help determine the applicable material in accordance with the working requirements, namely the stress state of an extraterrestrial structure. The compressive, flexural, and tensile strengths are the most commonly tested indicators. Almost all the extraterrestrial construction samples have been tested for compressive strength. For the impact resistance, 17 hypervelocity impact experiments were conducted by Allende et al. [35] to obtain the crater dimension in biopolymer concrete. The diameter of the impactor was approximately 1-3 mm, and the velocities ranged from 3-8 km∙s−1. Nano-hardness of molten regolith samples were tested by Zheng et al. [36], in which the nanoindenter applied a force of 0.1 N to the sample. Microhardness of the SPS, DLP, and laser-sintered samples was tested by other researchers using Vivtorinox hardness testers. The fatigue curve of “lunar cement” was obtained by Su et al. [37]. A Giga-Quad rotary-bending tester was used for the tests. The thermal and optical properties of construction materials have a significant impact on the livability and safety of buildings. A reasonable coefficient of thermal expansion (CTE) of a material can make the thermal stress of a structure acceptable. Accordingly, the CTE of microwave-sintered regolith samples were tested by Kim et al. [38]. The testing temperature ranged from −100 to 200 °C. In addition, good thermal insulation properties and proper light absorption performance can help create livable indoor conditions. The diffusivity heat capacity, and thermal conductivity of various sintered samples were tested by Fateri et al. [39]. The optical properties of the regolith powders and corresponding SPS samples were tested by Licheri et al. [28]. Radiation-shielding properties play an important role when considering extreme radiation in extraterrestrial space. Construction materials with good radiation-shielding properties can support long-term residency of astronauts. Montes et al. [40] conducted simulation to investigate the proton radiation shielding properties of geopolymer concrete. For neutron radiation shielding, the lunar regolith simulant samples were tested by Meurisse et al. [41]. Neutrons were produced by the spallation process of 800 MeV protons. For heavy ion radiation shielding, both the actual lunar regolith and regolith simulants have been tested [42]. Data from the above studies provides a valuable reference for determining the types of construction materials. The specific properties are shown in Fig. 1 [35], [36], [43], [44], [45], [46], [47], [48], [49], [50].
The published review papers on extraterrestrial regolith-based materials have mainly focused on the forming principle or preparation and construction process. However, a comprehensive review that analyzes and compares the construction-related properties of these materials is lacking. To bridge this research gap, the organization of characterization methods and the corresponding properties of extraterrestrial construction materials was conducted. The scope of the related paper database was determined in accordance with the topics organized in the preceding discussion on regolith-based construction materials. According to the bibliometric analysis of the database, the regolith and regolith simulation, occupy the most significant part of the database. The preparation methods for the construction process, such as microwave sintering, solar sintering, and digital light processing, are also core parts. In addition, the characterization methods for construction materials as well as the corresponding materials are the main components of the literature database, such as compressive strength, radiation shielding, and durability.
Based on the literature, different materials were compared, and their serviceability was analyzed. Challenges and prospects are also proposed. Thus far, construction materials have not yet been comprehensively characterized. Only a few studies have simulated complex and extreme extraterrestrial environments, and observations and characterizations were not performed in real time. High-throughput characterization methods that integrate various external load fields are recommended. The long-term performance of construction materials has not been sufficiently evaluated due to the lack of experimental conditions. Compared with long-term service after construction, the results obtained in a laboratory after hours of service are apparently not sufficiently credible. Consequently, accelerated tests are recommended. To characterize material properties in extreme extraterrestrial environments, experiments under real extraterrestrial scenarios are recommended.
The remainder of this paper is organized as follows. Section 2 lists and discusses common extraterrestrial construction materials. Section 3 presents and compares the mechanical properties of regolith-based materials, including their strength, impact resistance, hardness, and fatigue behavior. Section 4 presents the thermal and optical properties and discusses the heat absorption, light absorption, and thermal expansion behavior. Section 5 presents the radiation-shielding properties of common materials. Section 6 discusses the influencing factors and optimization methods for the long-term performance of a material. Section 7 presents the challenges and prospects for regolith-based materials. Finally, Section 8 summarizes this study.
2. Extraterrestrial construction feasibility and construction materials
2.1. Construction feasibility under extraterrestrial environment
To date, the Moon remains the most explored extraterrestrial body by humans. It is also regarded as a springboard for the exploration of Mars. Considering the huge economic cost of extraterrestrial transportation, the concept of construction based on in situ resource utilization is proposed. Owing to its accessibility, regolith is the primary raw material for extraterrestrial construction. With similar chemical components on Earth, the regolith on extraterrestrial bodies can also be cured as concrete and concrete-like materials or directly sintered and molten into bricks for construction. Moreover, the beneficial chemical components of modified admixtures, such as S and Mg, can be extracted in situ from the regolith to reduce the stress of extraterrestrial transportation. Regoliths can also be directly utilized as radiation- and thermal-shielding layers in structures. The existence of water/ice resources on Mars and the Moon has been researched for a long time. On the Moon, water/ice resources may exist in the polar regions [51]. On Mars, water resources are believed to be stored in the polar regions as well as in the ice layer beneath the surface soil at mid to high latitudes [52], [53]. The existence of water and ice resources also enables the preparation of water-containing concrete. Construction energy can be obtained from sunlight and mineral resources such as He-3. Additionally, combustible gases can be transformed from the atmosphere on Mars. Consequently, from the perspective of raw material acquisition, extraterrestrial construction based on regolith-based materials is highly feasible.
In addition to materials, specific construction processes have been proposed to take advantage of regolith-based construction materials and in-situ energy. As shown in Fig. 2 [54], [55], common extraterrestrial construction processes can be divided into two major types: prefabricated assembly [54] and three-dimensional (3D) printing [55]. In a prefabricated assembly, sintered regolith bricks and cured concrete are welded [56] or bonded to form a structure. The interlocking process is made optional by applying interlocking bricks [57]. By 3D printing, concrete or binders can be jetted to form the structures. Classical processes include D-shaping [58], [59] and contour crafting [60]. Regolith can also be directly sintered by some 3D printing technologies, such as selective laser sintering and solar sintering. Other construction processes such as regolith bagging have been proposed and developed. From the perspective of construction processes, the emergence of the aforementioned methods has made extraterrestrial construction feasible.
However, in contrast to the traditional construction processes on Earth, extreme extraterrestrial environments pose several challenges to extraterrestrial construction. Some of the harsh conditions are presented in Fig. 3. The transportation constraints limit the use of admixtures for construction materials. Low gravity, high temperature, and strong radiation may disrupt the forming process of construction materials. Temperature fluctuation and Moon/Mars quake may cause fatigue damage to building materials. Space debris may cause direct damage to the completed extraterrestrial structures. The property requirements of construction materials in extremely harsh extraterrestrial environments are different and higher than those on Earth. Consequently, it is necessary to evaluate and optimize the properties and corresponding performance of common regolith-based construction materials.
2.2. Extraterrestrial construction materials
Extraterrestrial regoliths can serve as primary raw materials for the production of construction materials. With the use of remote sensing and experiments on real samples, a certain understanding of lunar and Martian regoliths has been accumulated. Considering the limited applicability of actual extraterrestrial regolith samples as raw materials for destructive experiments, many regolith simulants have been developed by different research institutes. The preparation of regolith simulants is based on the existing understanding of actual regolith samples, and each has its own focused properties to be simulated. A summary of lunar and Martian regolith simulants is presented in Table 1 [8], [9], [10], [61], [62], [63], [64], [65], [66], [67], [68], [69], [70].
The particle size distributions (PSDs) of the samples collected from different locations on the Moon and Mars, as well as those of the regolith simulant, are shown in Fig. 4 [71], [72] and Table 2 [73]. For a comprehensive demonstration of the PSD, the data of 170 lunar regolith samples extracted using different sampling methods and from different missions are summarized. For a more concise representation, only the PSD boundaries of the samples from the different missions are marked. Taking the Apollo missions as an example, the coarsest one among the analyzed samples is Sample 66075.16 from Apollo 16, and the finest one is Sample 14141.30 from Apollo 14. Sample data from other missions, including the Chang’E-5 and Luna-24, are also shown. The PSD of the Martian regolith is shown in Fig. 4(b). The characteristic PSDs of the lunar regolith simulants are listed in Table 2, the range of which is within the boundaries of the real samples.
The chemical and mineral compositions of extraterrestrial regolith samples and their simulants are listed in Tables 3 [10], [61], [62], [63], [65], [66], [68], [70], [72], [73], [74], [75], [76], [77], [78], [79] and 4 [69], [80], [81], [82], respectively. The compositions of the samples from different exploration missions were similar, but also varied. A similar feature can be found in the samples, namely, the large proportion of SiO2, which indicates the potential of extraterrestrial regolith for the preparation of construction materials. For lunar regoliths, variations in the TiO2 content can be observed, which is also the criterion for determining high-Ti and low-Ti regoliths. It can also be observed from the tables that both lunar and Martian regolith simulants have compositions similar to the real ones. Simulants may be designed to serve different objectives, as listed in Table 1. For example, MLS-1 was designed to simulate a high-Ti mare regolith from Apollo-11, whose content of TiO2 is high above other simulants.
As stated earlier, concrete curing and material sintering/melting based on regoliths are the two most common processes used to form extraterrestrial materials. In concrete curing, the regolith is the primary component or aggregate and can be bonded by adding adhesives or undergoing polymerization reactions. Extraterrestrial concrete materials can be classified according to their binder compositions. The formation of cementitious concrete materials, such as Mg [21], [22] and silicate concrete [19], [20], relies on the cement hydration reaction [83]. Materials with binders provided by alkali activation are called geopolymer concretes [15], [16]. Materials with forming process that rely on the melting and solidification of binders to bond aggregates include polymer [17], [18], S [11], and biopolymer concretes [12], [13], [14]. For sintering and melting [84], [85], [86], regolith particles gather through heat and mass transfer. The forming process uses molds or additive manufacturing. Extraterrestrial sintering can be classified according to the energy resources, including solar light [87], [88], microwaves [24], [25], [26], [27], thermal radiation [23], electricity [89], ultraviolet (UV) [90], [91], [92], [93], and laser beams [29], [30], [31], [32], [33]. Other material-forming techniques, such as metal combustion [94], [95], spontaneous upward migration [96], cold sintering [97], dry aerosol deposition [98], directed energy deposition [99], slip casting [100], [101], and microbial-induced precipitation [102], are also available. The primary construction materials used in this study are shown in Fig. 5 [23], [34], [49], [103], [104], [105], [106], [107], [108], [110], [111]. Specific descriptions of common material processing methods are provided in Table 5.
3. Mechanical properties of regolith-based extraterrestrial construction materials
3.1. Compressive, flexural, and tensile strengths
After construction, the different components may be subjected to a range of pressures, tensions, and bending moments. To evaluate the reliability of structures, the corresponding mechanical properties of construction materials should be measured. Compressive, flexural, and tensile strengths were determined by conducting compression [113], three-point bending [113], and Brazilian tensile-strength tests [114], respectively. However, the specific parameters adopted in the experiments, such as the pressure rate and specimen shape, varied in different studies. For a comprehensive comparison, the shapes of the specimens and the corresponding standard bases in the mechanical property experiments of the extraterrestrial construction materials are summarized in Table 6 [36], [38], [49], [50], [112], [113], [114], [115], [116], [117], [118], [119], [120], [121], [122], [123], [124]. Cylindrical and prismatic specimens of different sizes were processed and tested according to the standards from different countries. Consequently, the comparison of the sample mechanical behavior among different studies may not be sufficiently reliable because the size and shape of a specimen affect the ultimate test results.
The specific values of the mechanical properties of the extraterrestrial construction materials are presented as Ashby plots in Fig. 6 [34], [36], [121], [123], [125], [126]. Only the samples whose compressive and flexural strengths were tested could be incorporated into the Ashby plots. Table 7 [14], [17], [18], [19], [23], [34], [36], [37], [47], [49], [50], [87], [102], [104], [105], [106], [109], [110], [112], [113], [114], [116], [122], [123], [124], [125], [126], [11], [127], [72], [128], [129], [130], [12], [131], [132], [133] lists the corresponding values. The following conclusions were drawn: First, the mechanical properties of the sintered samples were considerably better than those of the concrete samples, making them more suitable as load-bearing components. For example, the compressive strength varies from 0.90 to 76.80 MPa for the concrete samples, and from 2.31 to 428.10 MPa for the sintered samples. The flexural strength of the concrete samples varies from 2.30 to 47.80 MPa, and that of the sintered samples varies from 0.23 to 129.50 MPa. Second, magnesium concrete and DLP provide the best mechanical properties. In contrast, polymer concrete and solar sintering exhibit the worst mechanical properties. Third, the mechanical properties of the samples vary under different processing conditions. The most evident examples are the compressive and flexural strengths of the DLP samples. Suitable atmospheric environments and sintering temperatures can have significant effects. Consequently, setting the appropriate preparation parameters can considerably improve the performance of construction materials.
3.2. Impact resistance
In addition to the regular strength requirements for construction components discussed above, impact resistance should also be considered, given that micrometeorite impact is an extreme condition in extraterrestrial environments. Hypervelocity impact tests were conducted at NASA’s White Sands Testing Facility using a 0.17-caliber two-stage light-gas gun [134], as shown in Fig. 7(a) [134]. The tested samples are shown in Fig. 7(b) [135]. An increasing damage can be observed with an increase in impact energy. The experimental craters in the samples were 3D scanned [135] for further analysis. Those with radial cracks cannot be directly utilized, owing to possible boundary interference. A simulation model for hypervelocity impact experiments was built by Allende, which was based on a shock physics code developed by US Sandia National Laboratories [134]. The simulation model successfully simulates real experiments and makes it possible to conduct simulations for extrapolated parameters such as projectile sizes and velocities. Analytical power-law relationships can be derived [35] and crater dimensions can be predicted from the projectile features. The diameter, density, and velocity of the projectile can be used as inputs to the model, and the volume and diameter of the crater can be predicted. The developed model can predict the experimental data and simulated impacts with good accuracy; thus, it can serve as a design tool for optimizing the impact resistance of materials. However, only biopolymer concrete has been tested for impact resistance; data regarding other materials are lacking.
3.3. Hardness
Hardness refers to the ability of a material to resist hard objects pressing locally onto its surface. It is a comprehensive indicator of mechanical properties, such as elasticity, plasticity, strength, and toughness. The microhardness and nano-hardness [136] were tested for extraterrestrial samples prepared using different processes. In accordance with the statistical results presented in Table 8 [34], [36], [110], [121], [122], [137], [138], [139], a similar trend in hardness was observed between the sintered and molten samples. The hardness of the samples varied under various processing conditions. Notably, the microhardness of the DLP samples varied significantly depending on the sintering conditions. The hardness of samples sintered in air at 1150 °C was 33.41 times greater than that of the samples sintered at 1150 °C in Ar [34]. Unfortunately, hardness indices for concrete-like materials have not been reported in previous studies.
3.4. Fatigue behavior
When construction is conducted in extraterrestrial conditions, assessing long-term performance is crucial, particularly for permanent structures. One of the factors that affects long-term performance is the fatigue behavior of materials. Fatigue strength refers to the maximum stress at which a material will not fail under infinite alternating loads. Material fatigue originates from the changes in internal stress, which can result from an external force or thermal stress due to temperature fluctuations. With regard to outside force, the fatigue behavior of “lunar cement,” namely, the ultralow-binder-content inorganic-organic hybrid based on lunar soil simulant, was measured by Su et al. [37], as shown in Fig. 8 [37]. A ductile trend was observed when Johnson Space Center 1A (JSC-1A) was used as the filler, and the final failure occurred as a cleavage fracture. Fatigue resistance is higher than that of steel reinforcement in concrete, making “lunar cement” a trustworthy material for extraterrestrial construction.
With regard to the fatigue behavior under large temperature fluctuations in extraterrestrial conditions, the strength of various materials after a temperature shock was tested. According to Williams et al. [140], the temperature at the Moon’s equator varies from 96 to 397 K, and 50-200 K in the polar regions. Consequently, the influence of thermal stress should be considered to guarantee the performance of extraterrestrial construction materials. Three types of thermal fatigue behaviors can be observed in extraterrestrial construction materials, as shown in Fig. 9 [49], [124]. The strength could be unchanged, decreased, or increased. According to the experimental results reported by Lauermannová et al. [124], the strength of regolith-based Mg oxychloride composites made with fibers remained nearly unchanged after three heating/freezing cycles from −58 to 150 °C. For geopolymer concrete made from lunar regolith simulant, the flexural strength reduced by 45%-70% and the compressive strength reduced by 15%-19% after a temperature shock, simulating the temperature changes on the Moon [49]. In contrast, the compressive strength of a lunar geopolymer made by Pilehvar et al. [141] increased with freeze-thaw cycles. A possible reason for this is the continuous formation of an amorphous aluminosilicate gel. However, to date, no sample has been placed in lunar-simulated environments for more than one year. Only the short-term performance has been evaluated, and the long-term performance remains unknown. Moreover, the thermal fatigue properties of sintered samples have not yet been reported.
4. Thermal and optical properties
When working in a challenging extraterrestrial environment, extreme temperature changes can cause problems during the application of construction materials. Thus, the requirements for thermal and optical properties are accordingly designed. The thermal insulation and deformation properties should be considered. First, good thermal insulation of construction materials is required to provide appropriate living conditions inside the structure. In particular, materials should exhibit good heat insulation performances, such as low thermal conductivity and diffusivity. Poor thermal emissivity toward the interior of the structure is preferred to avoid internal thermal radiation. Second, large temperature fluctuations in extraterrestrial environments can lead to thermal expansion and contraction of construction materials, producing variable thermal stresses inside the structure. Variable and large expansions and contractions should be avoided because the resulting thermal fatigue affects the safety and stability of the structure [142]. Hence, structures should be composed of materials with low and similar CTEs. Third, the exterior temperature of a structure can be high because of heat transfer from the outside environment and the absorption of sunlight during daytime. Therefore, construction materials with low light absorption are sometimes recommended to avoid high exterior temperatures. To discuss the thermal and optical properties of materials, specific quantitative indicators are summarized in the following sections.
4.1. Thermal properties
Construction materials with good thermal properties should be able to withstand large temperature changes from the external environment and maintain internal temperature stability within a structure. Consequently, heat transfer from the outside to the inside of the structure is not encouraged. Heat transfer is achieved through thermal convection, conduction, and radiation. Thermal convection [143] refers to the heat transfer caused by the relative displacement of particles in a fluid. Thermal convection cannot occur in a vacuum extraterrestrial environment such as that on the Moon owing to the lack of fluid material. Consequently, discussions on the thermal properties of extraterrestrial construction materials are largely concerned with thermal conduction and radiation. During thermal conduction, many molecules in a substance collide with each other, causing energy to be transferred from the high-temperature part of the object to its low-temperature part. Thermal radiation refers to the phenomenon in which objects radiate electromagnetic waves owing to their temperature. In extraterrestrial construction materials, thermal conduction can occur inside dense materials, whereas thermal radiation can occur in material pores.
Overall, the thermal conductivity, thermal diffusivity, and heat capacity are common properties of concern for extraterrestrial construction materials. Good thermal insulation materials should have low thermal conductivity and high specific heat capacity. The thermal conductivity represents the amount of heat transferred through a material across unit length under stable heat transfer conditions, with a unit temperature difference on both sides of the surface, through a unit area within a unit of time. Thermal diffusivity is a measure of the rate at which temperature disturbances at one point in an object are transmitted to another. Specific heat capacity is the heat absorbed or released by a unit-mass object per unit change in temperature. All three properties are functions of temperature, and the correlations among the three can be expressed as
where T is temperature, K is thermal conductivity, α is thermal diffusivity, c is heat capacity, and ρ is density.
To comprehensively survey the thermal behavior of sintered lunar regolith materials, Fateri et al. [39] prepared a batch of samples using different processes from the JSC-2A lunar regolith simulant and tested their thermal properties, as shown in Fig. 10 [39]. Thermal diffusion measurements were conducted using laser flash analysis (LFA), and the specific heat capacity was measured using differential scanning calorimetry (DSC). The specific heat capacities of the tested samples were similar and increased with increasing temperature. Zheng et al. [36] discovered similar trends and values for a molten glassy lunar regolith. Least cubic fitting of the data yields
where cp is the specific heat capacity (J·(g·K)−1), and T is the absolute temperature (K).
The solar-sintered sample exhibited the lowest thermal conductivity and density, whereas the laser-molten sample exhibited the highest thermal conductivity. Song et al. [144] and Kost et al. [145] observed the relationship between pore formation and thermal conductivity. Song et al. [144] found that porous samples sintered at 1100 °C in a vacuum environment had relatively low thermal conductivity. Kost et al. [145] placed melting samples in a vacuum chamber and projected a laser to heat them. The temperature of the sample was recorded and simulated using MATLAB to determine its thermal conductivity. A similar behavior of density evolution with sintering temperature and thermal conductivity was observed. The thermal conductivity decreased with increasing porosity. Among the samples prepared by Fateri et al. [39], the oven-sintered samples, whose density was the highest among all the tested samples, demonstrated the lowest thermal diffusivity.
Repetitive thermal expansion and shrinkage can occur in extraterrestrial environments, leading to significant thermal stress and possible dehiscence. Therefore, elastic construction materials with similar CTEs are required. Kim et al. [38] investigated the thermal expansion of microwave-sintered lunar regolith samples (Fig. 11 [38]). Heating, cooling, and second heating were conducted to simulate the temperature fluctuations on the Moon. The rate of change in the sample length increased with increasing temperature. Almost no changes were observed between the first and second CTEs, confirming the high thermal resistances of the samples. The CTE value of the microwave-sintered sample was 5 × 10−6 °C−1, which was similar to that of lunar basaltic regolith breccias. More performance testing and discussions on the thermal expansion properties of extraterrestrial construction properties are encouraged to enrich the database and provide suggestions for the selection of the preparation process for materials.
4.2. Optical properties
The optical properties of extraterrestrial regoliths, including remote-sensing satellites and lander-based probes, have been widely studied during the exploration of the Moon and Mars. The prevalent data come from Visible/Near Infrared Spectrophotometer (VNIRS) on the American Lunar Crater Observation and Sensing Satellite (LCROSS), Visible and Infrared Mineralogical Mapping Spectrometer on the European Mars Express, and Imaging Interferometer (IIM) on China’s Chang’E-1, among others. Specifically, the reflection spectrum was investigated. The corresponding bands of the different detectors were not identical. Based on the feature analysis of different reflection spectrum data, mineralogical information, maturity, water content information, and other properties of the regolith can be learned. Strong forward scattering and significant impact effects of lunar regolith were observed [146]. The content of submicroscopic iron was obtained from spectral analysis and indicated the maturity of the lunar surface [147], namely, the degree of lunar regolith exposure in the spatial environment. In summary, the spectral features of the regolith result from the combined effects of mineral composition, space weathering, particle size, and geometry.
As far as structures are concerned, the optical properties of the material can affect the temperature of the structure. In addition to direct thermal transfer from the surrounding environment, the absorption of sunlight can also warm the structure and may cause uncomfortable indoor temperatures. Characterization of optical properties can help determine the ability of materials to absorb and emit sunlight [148]. When solar radiation is projected onto a material surface, some radiation is absorbed and some reflected. The solar energy absorption and reflection performance is expressed using the solar absorptance (alpha) and thermal emissivity (epsilon). The absorption rate of a material refers to the ratio of solar radiation energy absorbed to solar radiation energy projected onto the material. After absorbing solar radiation energy, the temperature of the material increases and the material radiates heat to the surrounding environment. The thermal emissivity of a material is the ratio of the thermal radiation energy emitted to that emitted by an absolute black body at a given temperature. Materials with selective absorption abilities exhibit good light absorption and thermal insulation [149], indicating high solar energy absorption rates and low thermal emissivities. Therefore, these materials are preferred as energy-storage materials [150], [151] in response to extraterrestrial protracted darkness [152]. This property can be quantified using the ratio of solar absorptance to thermal emissivity.
Licheri et al. [28] conducted related experiments to explore the optical properties of SPS samples prepared from lunar regolith simulant. Fig. 12(a) [28] shows the spectral absorptances of the powdered and sintered samples. The integral represents the solar absorptance of the sample. In terms of radiation trapping [153], the absorption spectrum is related to pore size distribution, leading to the difference between the curves of the samples sintered at 700 and 900 °C. After integration, the spectrum of the regolith simulant powder was calculated to be 0.77; that of the samples sintered at 700 °C was 0.88, and that of the samples sintered at 900 °C was 0.92. The sintered samples with higher spectra demonstrate the potential for use as solar energy receivers. The thermal emissivities and spectral selectivities of the samples are shown in Figs. 12(b) and (c) [28], respectively. The sintered samples exhibited a better selective absorption ability, making them appropriate energy-absorbing and storage materials. However, only a few studies have explored the optical properties of extraterrestrial construction materials, leading to a lack of comprehensive comparison and summary of rules.
5. Radiation-shielding property
Cosmic radiation has always been a critical obstacle in the exploration activities of astronauts because radiation exposure can cause biological damage [154]. The recommended ionizing radiation exposure limits are listed in Table 9 [155]. The major sources of cosmic radiation are solar particle events (SPEs) and galactic cosmic rays (GCRs) [45]. Owing to the lack of safety provided by the strong magnetic field and dense atmosphere of Earth, the strength of radiation in extraterrestrial environments is considerably higher than that on Earth. Solar cosmic rays are abrupt and unpredictable. The major components of SPEs are extremely intense protons [156]. Various types of heavy particles and ions from GCRs can accumulate in the bodies of astronauts for months and years. In addition to the protons and ions, neutrons are produced when GCRs interact with humans, spacecraft, and habitats [157], [158]. In summary, protons, neutrons, and heavy particles are the primary concerns regarding cosmic radiation.
Construction materials with good radiation-shielding properties can provide support for radiation shielding and help astronauts conduct exploratory activities. Various polymers and hydrides have been shown to be efficient in passive shielding, such as Kapton [45], lithium hydride [159], polyethylene [160], and Al [161]. However, the mass of the shielding materials poses a significant burden on mission costs. The in situ use of regolith-based shielding materials is a promising technique. Studies have been conducted using simulations and experiments to explore the radiation-shielding properties of regolith minerals [41], regoliths [42], and regolith-based materials [40].
5.1. Proton radiation shielding
Protons are positively charged subatomic particles with different amounts of energy that exist in SPEs and GCRs. Montes et al. [40] used the FLUktuierende KAskade (FLUKA) simulation tool to simulate particles based on the Monte Carlo method, and demonstrated the proton shielding properties of geopolymer concrete. The geopolymer concrete shielding material was prepared with a JSC-1 lunar regolith simulant and the habitat was simulated as a sphere 460 cm in diameter. The thicknesses of the shield were set to 50 and 100 cm. To represent protons in different events, 40, 100, and 400 MeV monochromatic protons were introduced in the simulations. The simulations analyzed the absorbed dose and dose-equivalent quantities in humans, and crew members (represented by squares inside the shielding structure in Fig. 13 [40]). The radiation-shielding properties are demonstrated in the form of a heat map in Fig. 13, where deep red represents high deposited energy and deep blue represents low deposited energy. Based on the simulation results, it can be concluded that the geopolymer concrete with a depth of 50-100 cm can provide sufficient shielding protection for the long-term residence of astronauts. Special protection should be designed for the rare, high-energy protons generated in solar flares.
5.2. Neutron radiation shielding
Neutrons are uncharged elementary particles that are generated when primary particles collide. As shown in Fig. 14 [41], powders and sintered samples of lunar regolith simulant were used in the neutron radiation-shielding experiments. An experiment on neutron radiation was conducted at the ChipIr facility. Neutrons were generated using 800 MeV protons. The neutron beam was incident on the regolith and its flux measured using an Si radiation detector shown in Fig. 14(a) to record radiation attenuation. JSC-2A powder, vacuum-sintered JSC-2A, and solar-sintered JSC-2A samples were used for testing. The experimental results are shown in Fig. 14(b). Lunar regolith had a neutron-shielding capability similar to that of Al. The experiment validated the pre-set Monte Carlo radiation simulation, as shown in Fig. 14(c). Based on the simulation results, 200 g∙cm−2 of sintered JSC-2A samples can be helpful for GCR neutrons, which refers to 80 cm of sintered regolith or 150 cm of loose regolith.
5.3. Heavy ion radiation shielding
Heavy ions, such as 56Fe, 48Ti, 40Ar, 28Si, 20Ne, 16O, 14N, 12C, and 10B [162], are important components of GCRs. To investigate the radiation properties of construction materials for heavy ions, Miller et al. [42] collected several real lunar regolith samples from Apollo and typical lunar regolith simulants and conducted corresponding radiation measurements. The radiation measurements were performed in two phases. In the first phase, a higher beam energy of 400 MeV per nucleon was used. The regolith and simulant were tested for comparison with Al, polyethylene, and graphite. The shielding effect of the lunar regolith simulant was nearly equivalent to that of Al but only half that of polyethylene. In the second phase, the energy of the beam was set to 290 MeV per nucleon, and the experimental objects were regolith simulants, such as NU-LHT-1. The Bragg curves from the tests are presented in Fig. 15 [42]. Attenuation of some representative components of GCRs were observed [163]. Less than half a meter of regolith, with a density of 1.4 g∙cm−3, is sufficient for stopping primary GCR ions.
In summary, regolith-based materials can shield against protons, neutrons, and heavy-particle radiation. For untreated lunar regolith, 46 cm or less of compacted powder [42] can protect astronauts from GCR nuclei and SPE protons. A loose regolith of 150 cm [41] can protect astronauts from GCR neutrons. A 50-100 cm thick regolith-based geopolymer concrete [40] provides shielding from proton radiation. The shielding performance of sintered regolith is similar to that of Al, and a layer of 80 cm can significantly improve the GCR and SPE neutron shielding [41].
6. Influencing factors and optimization methods for the performance of regolith-based materials
The above discussion on the mechanical, thermal, and radiation-shielding properties of extraterrestrial construction materials can help identify appropriate types of materials for practical applications. A comparison of the strengths and weaknesses of these materials is presented in Table 10.
The factors influencing these properties should be analyzed to optimize the preparation of materials with better performance. Although the influencing factors can vary depending on specific forming processes, some common issues can be dealt with as discussed next.
6.1. Influencing factors
6.1.1. Influencing factors for mechanical and thermal properties
The most critical factor affecting the mechanical and thermal properties of construction materials is their microstructure, which is pores and cracks. A general law can be observed that the higher the porosity, the weaker is the mechanical property. In contrast, the higher the porosity, the better is the thermal insulation performance. This law applies to extraterrestrial construction materials, including sintered [110], [144], [145] and concrete-like [103] materials, as shown in Fig. 16 [110], [145]. From the perspective of long-term performance, existing pores and cracks can develop in extreme service environments, as shown in Fig. 17 [49], [118]. However, only the effect of temperature changes on the microstructural development of the materials has been investigated. Data on radiation and other harsh environmental conditions have not yet been reported.
The generation of pores and cracks is attributed to various factors. First, the components and morphology of the raw materials can significantly affect the microstructures of the samples. As the most important raw material, the components of the regolith significantly influence the final samples. Ilmenite is the most prevalent component studied by researchers, for the existence of both high- and low-Ti regoliths on the Moon. Song et al. [137] reported a possible ion substitution between ilmenite and other minerals during vacuum sintering. Zhou et al. [133] performed microwave sintering of a regolith containing 4.6% ilmenite by weight and demonstrated its superior density and mechanical performance. Meurisse et al. [164] found that the optimal sintering temperature for De NoArtri 1 (DNA-1) was lower than that for JSC-1A owing to their different mineral compositions. Specifically, the sodium feldspar mineral in DNA-1 has a lower melting point than the calcium-rich plagioclase in JSC-1A. Consequently, the samples prepared from DNA-1 exhibited a higher porosity and lower strength. According to the experimental results, the impact of the compositional difference between the simulated and actual regoliths should be considered when conducting related experiments. Except for the components of regolith, and morphology of regolith also matters. Collins et al. [116] found that geopolymer concrete prepared from JSC-1A exhibited a higher compressive strength than that prepared from JSC-2A. Despite the same feedstock, JSC-2A was milled and exhibited higher internal stresses; more spherical particles in JSC-2A can lead to a ball-bearing effect and, consequently, a more workable mixture. The mercury intrusion porosimetry (MIP) curves of the samples showed that those made from JSC-2A had a lower porosity. Except for the regolith, the components and addition of other raw materials can also affect the sample properties. Using a geopolymer concrete, orthogonal experiments were conducted by Zhou et al. [49] and Zhou et al. [113] to investigate the significance of the reactant ratio on the mechanical properties of the sample. The superfluous concentration of the alkaline solution was found to dissolve from the geopolymer gel, which could lead to volume expansion and cracks inside the structure, and consequently, weakened strength. They found that the addition of Al2O3 and Metamax effectively improved the mechanical strength of geopolymers, indicating the need for an adequate amount of Al during the reaction to avoid cracks [120]. Second, the preparation conditions can significantly affect the microstructure of the samples. Different open, closed, and total porosities were observed by Kim et al. [38] when a regolith was sintered in a microwave at different temperatures. Their compressive strengths and CTE differed accordingly. For additive manufacturing, a printing layer thickness that best matches the depth of UV curing can achieve the lowest porosity and the best compressive strength. When concrete is considered, the curing temperature can affect the porosity and cracks in the materials, leading to different mechanical properties [113]. Vacuum solidification [117] affects the progress of the reaction as the activated solution is discharged from the cube into the surrounding mold and then evaporated. Third, the service environment can alter the microstructure after preparation. Microgravity [165] can help generate unique microstructures when diffusion-controlled crystal growth is achieved. Temperature cycling [49], [118], [141] can generate microcracks and promote their development, thereby reducing the strength of the samples.
6.1.2. Influencing factors for radiation-shielding properties
With regard to radiation-shielding properties, the chemical components of the materials are the most important factors. The shielding effects of polymers and low-density hydrides were studied by Al Zaman et al. [45], where the materials were attached in a toroid shape with different thicknesses. Water is known to be an efficient neutron barrier; H slows down neutrons via elastic scattering. The addition of 1.0 wt% water to the lunar regolith simulant significantly affected the shielding properties. A wet lunar regolith with a thickness of 12 cm shields neutrons as much as a dry lunar regolith with a thickness of 22 cm and a density of 2.54 g∙cm−3 [41]. Some designs for extraterrestrial habitats have considered water as the inner layer, which is an economical and effective approach for radiation shielding [166], [167].
The degree of densification should also be considered in terms of radiation-shielding properties. As discussed in Section 5, the effective amount of radiation-shielding material is frequently expressed in terms of the surface density. Thus, the required thickness decreases when the degree of compaction increases. Sintering and polymerization can also help densify regolith powders and obtain better radiation-shielding properties.
6.2. Optimization methods
The optimization methods used in recent studies are listed in Table 11 [23], [38], [48], [49], [50], [104], [110], [113], [114], [116], [120], [122], [124], [125], [129], [132], [133], [168], [169]. Methods for controlling the variables have been used to optimize the properties of materials, and some studies have conducted systematic orthogonal experiments.
For the mechanical properties, optimization methods can be implemented in two ways. First, raw materials should be prepared appropriately. As discussed earlier, the ratio of the reactants should be optimized to avoid redundant raw materials and reduce the generation of cracks. Fibers can also be added as raw materials to fill the micropores and increase the strength of materials. The chemical components of the materials can be changed by adding admixtures. For example, with an increase in ilmenite content, the strength of microwaved samples improves owing to the good dielectric properties of ilmenite [133]. Second, the preparation parameters should be analyzed and optimized. For concrete and concrete-like materials, the curing temperature and pressure are significant parameters that should be optimized. For sintered and molten materials, the sintering temperature, heating rate, and heating pressure are important parameters that must be considered. Additional parameters can be optimized for additive-manufactured materials. Using selective laser melting as an example, the corresponding parameters include substrate type, hatch spacing, scanning speed, and laser power [122]. Appropriate preparation parameters can help to achieve well-formed and continuous laser tracks, thereby obtaining two-dimensional (2D) planes and 3D objects of good quality. Effective optimization of material preparation can considerably improve its properties.
The effect of the microstructure on the thermal properties is significant, and the preparation of high-porosity materials is a key factor for improving the thermal properties. A vacuum environment is a natural condition for obtaining sintered porous materials when substances evaporate in large quantities. In the experiments conducted by Song et al. [144], the weight loss ratio during vacuum sintering was approximately four times than that during air sintering. With regard to the radiation-shielding properties, the addition of water and the effective densification of regolith powders are the recommended methods based on the discussion in Section 6.1.2.
7. Challenges and prospects
The performance of commonly used extraterrestrial construction materials has been quantified by different researchers. However, various challenges exist that hinder their practical applications. The reliability of these materials for preparation and service in extreme environments is still not sufficiently high. Specific challenges and future directions are discussed below.
7.1. Challenges for regolith-based construction materials
7.1.1. Comprehensive characterization of extraterrestrial construction materials
Current testing experiments are mostly conducted in a traditional manner, in which the comprehensive properties of the materials cannot be obtained. The extraterrestrial service environments for construction materials are complex and coupled. For example, the large temperature fluctuations on the Moon do not lessen in the presence of solar wind. The performance of materials may differ under coupled loading, considering that strong radiation and temperature cycling can cause internal structural damage. Accordingly, for service performance characterization, load coupling should be considered. However, the comprehensive characterization under coupled loads is scarce in previous studies, and the application of only one load has been more prevalent. The lack of comprehensive characterization can hinder the knowledge of practical performance and thus influence the judgement for material selection or further optimization.
7.1.2. Long-term performance evaluation of extraterrestrial construction materials
For the construction of permanent structures, the long-term performance of materials should be investigated. The existing research on the performance of extraterrestrial construction materials remains at the laboratory level; thus, the service time of these materials is only a few dozen hours. The long-term performance characterization of extraterrestrial construction materials conducted in previous studies is provided in Table 12 [49], [117], [124], [141]. For the temperature cycles, the maximum number of rounds was 40. For the curing period, the maximum maintenance time was 28 d. Such short-term performance characterization cannot prove the applicability of materials. Only the performances of geopolymer concrete and magnesium concrete have been tested under extreme temperatures and hard vacuum; long-term effects of other harsh conditions remain unknown. For the properties, only the strength has been considered.
7.1.3. Characterization under extreme extraterrestrial environment
The challenges for extraterrestrial construction materials mostly lie in extreme working environments. However, the creation of extreme extraterrestrial environments on Earth is still difficult because of technical bottlenecks and cost constraints. Taking radiation and micrometeorite impacts as examples, these two loads are usually simulated instead of being practically applied. Hence, in most cases, the actual performance of materials is derived using simulation models instead of direct testing. Another example is the temperature cycling. According to previous research and the summary in Table 12, temperatures created in the laboratory are not always as extreme as those on the Moon and Mars. Limited by the realization of extreme extraterrestrial environments, the real properties of these materials remain ambiguous.
7.2. Prospects for regolith-based extraterrestrial construction
7.2.1. High-throughput characterization under multiphysics
For comprehensive characterization, the application of high-throughput characterization methods under multiphysics is recommended for future research, which will considerably help in improving traditional testing methods. High-throughput methods [170], [171] aim at rapid, refined, and accurate characterization to obtain a large amount of data, whereas multiphysics is applied to simulate practical situations. This combination can be realized by integrating an external load into the characterization device, which can be studied further. When the service environment of a material is a complex loading condition coupled with multiple fields, it is frequently accompanied by damage phenomena, such as corrosion, embrittlement, creep, and fatigue, which seriously affect the performance and life of a material. The failure mechanism is also significantly different from research results under standard test conditions, highlighting the need for proper characterization methods. The basic idea of fine characterization of the internal response of materials under multiload field coupling is to develop specialized devices that integrate various external load fields and ultimately achieve high-throughput acquisition of microstructural damage evolution information and performance parameters of disciplines, such as mechanics and spectroscopy. For example, multidimensional and multiscale in situ characterization technologies based on advanced light sources such as synchrotron light sources and spallation neutron sources can realize 3D full-field and in situ dynamic characterization of internal microstructures with high density, large size, and bearing capacity.
7.2.2. Accelerated durability test based on ground simulation
Regarding long-term performance evaluation, the challenges lie in the time and economic costs of long-term extraterrestrial environment simulation. The same challenges exist when evaluating construction materials on Earth, for which accelerated tests are proposed and widely applied. The durability of concrete and other construction materials has been extensively studied using accelerated tests. Yang et al. [172] concluded that 270 d exposure to constant temperature alkaline solution at 60 °C was equivalent to 50 years of actual service. The basic guiding ideology is predicting the average durability under normal influencing factors based on average durability. Similarity theory indicates that if two phenomena are similar, the relationships between several parameters describing the phenomena can be transformed into functional relationships between similarity criteria, and the similarity criterion function relationships of similar phenomena are identical [173]. By studying the similarity theory and determining the influencing factors through a pre-conducted test, the similarity criteria to bridge the experimental simulation conditions and real environment can be acquired. Accelerated testing is recommended for extraterrestrial service scenarios. Prediction equations or models may be obtained from the results of accelerated tests using similarity analysis, and the long-term performance can be obtained accordingly.
7.2.3. Experiments under real extraterrestrial scenarios
Considering the difficulties of creating extreme environments on Earth, experiments under real extraterrestrial scenarios are recommended. In situ forming and testing based on in situ materials is suggested. Nearly all experiments conducted in previous studies were based on the International Space Station. However, these experiments did not focus on regolith-based construction materials. For example, space-exposure experiments have been conducted using biomaterials [174] and solid lubricating materials [175]. Liquid sintering behavior has also been studied for metallic materials [176]. The feasibility of extraterrestrial experiments has been demonstrated by these projects. For future directions, space station experiments on regolith-based construction materials should be the objective of researchers, such as forming experiments under micro- or zero-gravity and exposure experiments for formed materials. In addition, in situ forming and testing equipment can be considered for lunar exploration and flight missions. In situ forming experiments can be conducted when the equipment is developed and taken into space. The specimens could be exposed to the actual extreme environments for property characterization. In addition, the establishment of the International Lunar Research Station and Lunar Camp can provide unprecedented strong support, where lunar regolith can be used to form a specimen, and testing experiments can be conducted in lunar environment.
8. Conclusions
To provide a clear perspective on the development of extraterrestrial construction materials and propose future research directions to improve their feasibility, a comprehensive survey was conducted on the properties and characterization of regolith-based materials. Studies on extraterrestrial concrete, concrete-like materials, and sintered/molten materials were collated and analyzed, and the specific construction-related properties of 13 types of extraterrestrial construction materials were summarized and compared.
With regard to extraterrestrial construction, the extremely harsh processing and service environment should be considered, bringing attention to three specific types of properties: mechanical, thermal and optical, and radiation-shielding. Discussions on the mechanical properties facilitate the analysis and comparison of the reliability and applicability of different materials. The strength, impact resistance, hardness, and fatigue behavior are correspondingly organized. The sintered samples exhibit better mechanical properties than the concrete samples. Analysis of the thermal and optical properties emphasizes the thermal insulation effect and the resulting thermal stress of the materials. Materials with low thermal conductivity and high specific heat capacity are more likely to provide a suitable indoor environment, whereas materials with similarly low CTEs can lead to low structural internal stress. However, experimental data in this area remain limited for a comprehensive comparison. Simulation and experimental results have been obtained by previous research to explore the radiation-shielding properties of construction materials. Densifying the in situ regolith through compaction and sintering is useful for better radiation shielding performance. The addition of water also significantly helps with radiation shielding. In addition to organizing and summarizing the properties of extraterrestrial construction materials, the influencing factors and optimization methods for performance were analyzed. Good preparation of raw materials and appropriate methods for determining the optimal parameters during forming can help considerably with the performance of materials.
However, challenges remain and hinder the prospects of extraterrestrial construction materials. The first challenge concerns the shortcomings of traditional characterization methods, which lead to incomprehensive characterization. Accordingly, high-throughput characterization methods in multiphysics are suggested. The second challenge is the evaluation of long-term performance under limited experimental conditions. Therefore, the application of accelerated tests is suggested. Conducting real extraterrestrial experiments should be considered. The development of space stations can provide significant support for real extraterrestrial experiments.
Acknowledgments
This work was supported by the National Key Research and Development Program of China (2023YFB3711300 and 2021YFF0500300), the Strategic Research and Consulting Project of the Chinese Academy of Engineering (2023-XZ-90 and 2023-JB-09-10), and the National Key Research and Development Program of China (2021YFF0500300).
Compliance with ethics guidelines
Cheng Zhou, Yuyue Gao, Yan Zhou, Wei She, Yusheng Shi, Lieyun Ding, and Changwen Miao declare that they have no conflict of interest or financial conflicts to disclose.
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